1. Field of the Invention
This invention relates to a polyimide material and its method of manufacture. Particularly this invention relates to a fully imidized cresylic acid soluble polymer prepared from 4,4'- oxydiphthalic anhydride, 3,4,3',4'-biphenyltetracarboxylic dianhydride, 3,4'-oxydianiline, and 4,4'-oxydianiline.
2. Description of the Invention Background
Typical commercial copolyimides are known to perform well as electrical conductor insulators, and therefore act as adhesives, coatings, or films over high temperature electrical conductor materials such as, for example, copper, aluminum, titanium substrates, ceramic, and glass. For instance, magnet wire, for use with manufacturing motors, coils, transformers, and magnetic components, is coated with a copolyimide (wire enamel) in one operation, subsequently wound into motors and transformers, and then dipped, impregnated or trickled with an insulating varnish in another operation. The varnishing step in typical insulating conductor processes necessarily provides mechanical strength and improved electrical properties to the wire. Typical wire enamels and insulating varnishes work in conjunction with other materials to form a complete electrical insulating system. The varnishing step, however, adds significant time and cost to the manufacturing process.
Coating an electrical conductor with copolyimide may be accomplished in a number of ways, such as, for example, through dip coating. The typical polyimide wire enamel is applied to magnet wire from a nitrogen containing solvent in the form of a polyamic acid. In dip coating, the electrical conductor is unwound from a roller at a selected wire speed, passed into a bath of the wire enamel resin, and through a curing tower that converts the polyamic acid to the polyimide state. The curing tower temperature is set sufficiently high, given various factors such as wire speed, to heat the polyimide coating above its glass transition temperature (Tg) in order to drive off solvents.
Due to the insolubility of fully imidized wire enamels, from a practical standpoint, polyimide wire enamels are available only in the polyamic acid state and are soluble only in nitrogen containing solvents, such as N-methyl pyrrolidone. Typical nitrogen solvents used for copolyimides are relatively expensive. In addition, when exposed to air, nitrogen solvents, other than cresylic acid, absorb water and tend to precipitate resin from the solution.
It is known to use 4,4'-oxydiphthalic anhydride and 3,4,3',4'-biphenyltetracarboxylic dianhydride to form copolyimides having specific properties and uses. U.S. Pat. No. 5,171,828 to Meterko et al. discloses copolyimides having high comparative tracking indexes to be used as an electrical insulator formed from 4,4'-oxydiphthalic anhydride, 3,4,3',4'-biphenyltetracarboxylic dianhydride, and 4,4'-oxydianiline or para-phenylenediamine, wherein the copolyimides are applied as a polyamic acid prior to imidizing, and not as an imidized solution.
U.S. Pat. No. 5,639,850 to Bryant discloses a relatively tough wholly aromatic, thermoplastic polyamide copolymer prepared by combining 4,4'-oxydiphthalic anhydride, 3,4,3',4'-biphenyltetracarboxylic dianhydride, and 3,4'-oxydianiline that is soluble in common amide solvents such as, N,N'-dimethyl acetamide, N-methylpyrrolidinone, and dimethyl formamide, and can be applied as a fully imidized copolymer in addition to the amic acid solution. The copolyimides in Bryant, however, have relatively high viscosities in the workable solution that reduces the single pass effectiveness in the enameling process.
Accordingly, the need exists for a copolyimide that can be used in the wire enamel coating process which is a fully imidized cresylic acid soluble polymer that has relatively low viscosity and a high percent of solids for good bonding, insulating, and layering characteristics and which may be self-bonding in order to eliminate the varnishing step in insulating conductor processes.